Foods are nowadays increasingly frequently supplied in plastics packaging which encompasses a packaging tray into which the foods are placed and a lid foil with which the packaging tray is sealed.
The packaging trays are usually produced by thermoforming, mostly by deep-draw-thermoforming, from a thermoformable multilayer foil, with exposure to heat. After introduction of the product for packaging, the sealing of the packaging tray with the lid foil usually takes place with exposure to heat, i.e. by heat-sealing.
There is known packaging of this type in which the lid foil is composed of a heat-shrinkable multilayer foil.
Heat-shrinkable multilayer foils are usually biaxially oriented foils, being used in the packaging of foods, more particularly of perishable foods, such as poultry or fresh meat, where these multilayer foils preferably also have a barrier layer which is impermeable to gases and/or to flavors. Heat-shrinkable multilayer foils have the property of shrinking back to their original, unoriented dimensions when they are heated to temperatures above 60° C. Biaxially oriented multilayer foils are foils oriented both longitudinally and transversely, and they often have both longitudinal and transverse heat-shrinkability (shrinkage capability) greater than or equal to 35%.
There is a requirement for packaging in which the foil web from which the packaging elements, preferably the packaging trays, are produced is a heat-shrinkable multilayer foil with very good mechanical properties, so that the packaging elements produced therefrom, preferably packaging trays, also have excellent mechanical strength, more particularly puncture resistance, inter alia in order to provide safe handling of packaging produced therefrom, and where the excellent heat-shrinkability of the multilayer foil is in essence retained after the forming process and sealing process for packaging. However, this is difficult to achieve. By way of example, it is difficult to achieve the thermoformability required in a multilayer foil in order to form the packaging tray while at the same time obtaining adequate heat-shrinkability, more particularly in the thermoformed regions.
The invention is therefore based on the object of providing a multilayer foil preferably in the form of a packaging material which has advantages over the multilayer foils of the prior art. More particularly, the multilayer foil should be capable of thermoforming, preferably deep-draw-thermoforming, to give a packaging element, preferably a packaging tray, and, after introduction of the product for packaging, be capable of sealing with a further packaging element, preferably a lid foil, without any significant shrinkage of the multilayer foil before these processes are completed. The intention was that, once the sealing process has been completed, the shrink process can be induced via conventional measures, for example via exposure to heat, the result being that the multilayer foil in the form of packaging material is at least to some extent in close contact with the product for packaging. The intention here was that, after the thermoforming process and the heat-sealing process, there should still be adequate heat-shrinkability present, alongside excellent mechanical strength.